In recent years, vehicles are equipped with various assisting systems to reduce fuel consumption and make travelling of the vehicles smooth. A power steering system is one of such systems. The power steering system makes driving of a vehicle comfortable and makes driving operations easy and accurate.
Such power steering systems include an electric power steering system (hereafter, referred to as EPS system) which directly assists the steering operation with a motor (electric motor), an electro-hydraulic power steering system (hereafter, referred to as EHPS system) which assists the steering operation by supplying hydraulic pressure with an electric pump, and the like.
Since both systems can assist the steering operation with the motor which operates irrespective of an engine, the fuel consumption of the vehicle can be improved. Particularly, the EHPS system can provide operation feeling similar to that of a conventional hydraulic power steering system and is thus often provided in heavy-weight vehicles such as trucks.
A control method of the EHPS system is described with reference to FIGS. 5 and 6. As shown in FIG. 5, an EHPS system 10 includes an electric pump 11 having a hydraulic pump 11a and a motor (electric motor) 11b, a reserve tank 12, a power steering unit 13 (steering assisting device) 13, a relief valve (line pressure control valve) 14, and a check valve 15.
Generally, the control of the EHPS system 10 is control of increasing and decreasing assisting force of the power steering unit 13 by supplying fluid (hydraulic oil) from the electric pump 11 to the power steering unit 13. It is possible to reduce energy consumed by the electric pump 11 by causing the electric pump 11 to supply the fluid when the assisting force of the power steering unit 13 is required and by stopping the supply of fluid when the assisting force is unnecessary, i.e. when the steering wheel is at a neutral position in the steering operation.
However, when the supply amount of fluid is set to zero, there is a risk of a seizure phenomenon occurring in the power steering unit 13, a wobbling phenomenon of the steering wheel occurring due to disturbance such as kickback in straight travelling, or a similar phenomenon occurring. Furthermore, setting the supply amount to zero has a problem that, in abrupt steering or the like, a long time is required for the supply amount of fluid to increase, and response delay occurs in the steering operation assist. In view of this, in cases such as where no steering operation is required, the electric pump 11 is controlled to supply the fluid to the power steering unit 13 at a minimum required standby flow rate.
Specifically, as shown in FIG. 6, first, a standby rotation speed N0 (rpm) of the electric pump 11 corresponding to the vehicle speed Vn (km/h) is basically defined by using a standby rotation speed map M1. In cases such as where no steering operation is required, the rotation speed of the electric pump 11 is controlled to reach the standby rotation speed N0.
When the steering wheel is operated, a vehicle speed coefficient (also referred to as vehicle speed responsiveness) k corresponding to the vehicle speed Vn is calculated from a vehicle speed coefficient map M2. Moreover, an additional rotation speed N1 (rpm) corresponding to a steering speed ωn (deg/s) is calculated from a steering speed sense map M3.
Next, the calculated vehicle speed coefficient k and the calculated additional rotation speed N1 are multiplied by each other, and a steering correction value N1′ (rpm) is thereby calculated. Then, the standby rotation speed N0 and the steering correction value N1′ are added to each other, and a target rotation speed N2 (rpm) is thereby calculated. Thereafter, the rotation speed of the electric pump 11 is controlled to reach the target rotation speed N2.
However, the aforementioned control method has a problem that, when the vehicle is travelling on a slope, the steering force is insufficient in slope descent due to increase in a front axle load and steering is difficult. The aforementioned control method also has a problem that steering force is excessive in slope ascent and feeling of response is lost.
Regarding these problems, if the standby rotation speed N0 is increased to overcome the problem of insufficient steering force in slope descent for example, the electric energy of the electric pump 11 increases and the fuel consumption deteriorates. Moreover, the increase of the standby rotation speed N0 leads to increase of excessive steering force in slope ascent. Meanwhile, if the standby rotation speed N0 is reduced to overcome the problem of excessive steering force in slope ascent, the steering operation becomes difficult or impossible in slope descent due to insufficient steering force.
To counter the aforementioned problems which occur when the vehicle is travelling on a slope, there is a device which selects a gear ratio corresponding to a gradient of a road surface to achieve a good balance between turning performance in slope ascent and stability in slope descent (for example, see Patent Document 1). As in this device, in a vehicle equipped with the EPS system, increase and decrease of assist force from the motor is directly transmitted to the steering wheel. Accordingly, even if the road is sloped, the steering operation can be immediately assisted by increasing or decreasing the assist force in the steering operation.
Meanwhile, in the EHPS system, if the supply amount of fluid from the electric pump is increased and decreased according to the gradient of the road surface in the steering operation as in the aforementioned device, a long time is required for the fluid to be supplied from the electric pump to the power steering unit, and responsiveness to the steering operation deteriorates.
Particularly in a vehicle such as a cab-over truck, a front axle load which is the greatest factor of steering force changes greatly due to a short wheelbase and increase in the height of the center of gravity which is caused by loading of cargo. Accordingly, there occurs jerky feeling in the steering operation due to insufficient steering force in slope descent and lack of response in the steering operation due to excessive steering force in slope ascent. Hence, the assist force in the steering operation needs to be increased and decreased according to the gradient of a road surface and there is a need to consider control according to the gradient of the road surface.